Minghe

Minghe

Welcome To Sand Casting Org

Minghe Sand Casting Org Is Professional Sand Casting Encyclopedia Base.

Relevant Information

Casting method in which molding sand and core sand are used as molding materials to make a mold, and liquid metal is filled with the mold under gravity to produce a casting. Steel, iron and most non-ferrous alloy castings can be obtained by sand casting methods. Because the modeling materials used in sand casting are cheap and easy to obtain, and the molds are easy to manufacture, they can adapt to single-piece production, batch production and mass production of castings. For a long time, it has been the basic process in casting production.

The mold used in sand casting is generally composed of a combination of an outer sand mold and a core. In order to improve the surface quality of castings, a layer of paint is often applied to the surface of the sand mold and core. The main components of the coating are powdery materials and binders with high refractoriness and good chemical stability at high temperatures. In addition, a carrier (water or other solvents) and various additives are added for easy application.

Raw Materials

The basic raw materials for making sand molds are foundry sand and molding sand binder. The most commonly used foundry sand is siliceous sand. When the high-temperature performance of silica sand cannot meet the requirements of use, special sand such as zircon sand, chromite sand, and corundum sand are used. In order to make the finished sand mold and core have a certain strength and not be deformed or damaged during handling, molding and pouring of liquid metal, it is generally necessary to add a molding sand binder in the casting to bond the loose sand particles together to form molding sand. The most widely used molding sand binder is clay, and various drying oils or semi-drying oils, water-soluble silicates or phosphates and various synthetic resins can also be used as molding sand binders. The outer sand molds used in sand casting are divided into three types: clay green sand, clay dry sand and chemical hardened sand according to the binder used in the sand and the way it builds its strength.

Clay Wet

Clay green sand molds use clay and an appropriate amount of water as the main binder for molding sand. After the sand mold is made, it is directly combined and poured in a wet state. Wet casting has a long history and is widely used. The strength of green sand depends on the clay slurry formed by mixing clay and water in a certain proportion. Once the molding sand is mixed, it has a certain strength. After being pounded into a sand mold, it can meet the requirements of molding and pouring. Therefore, the amount of clay and moisture in the molding sand are very important process factors.

The advantages of clay green sand casting are:

  1. Clay is rich in resources and low in price.
  2. Most of the used clay wet sand can be recycled and reused after proper sand treatment.
  3. The cycle of manufacturing the mold is short and the work efficiency is high.
  4. The mixed molding sand can be used for a long time.
  5. After the sand mold is pounded, it can still tolerate a small amount of deformation without being damaged, which is very beneficial for drafting and core setting.

The disadvantages are:

  1. To coat the viscous clay slurry on the surface of the sand grains during sand mixing, high-power sand mixing equipment with rubbing action is required, otherwise it is impossible to obtain good quality sand.
  2. Since the molding sand has very high strength after being mixed, the molding sand is not easy to flow during modeling, and it is difficult to pound. It is laborious and requires certain skills when modeling by hand, and the equipment is complicated and huge when modeling by machine.
  3. The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor.
  4. Castings are prone to defects such as sand washing, sand inclusion and pores.

At the beginning of the 20th century, the foundry industry began to use roller-type sand mixers to mix sand, which greatly improved the quality of clay green sand. The new high-power sand mixer can achieve high efficiency and high quality in sand mixing work. The emergence of the shock compaction molding machine, which mainly focuses on compaction, has significantly improved the compactness and uniformity of the mold. As the requirements for the dimensional accuracy and surface quality of castings increase, high-pressure molding machines that mainly compaction have appeared. The use of a high-pressure molding machine to make clay green sand molds can not only improve the dimensional accuracy of the castings and improve the surface quality, but also simplify the action of the compact mold and shorten the cycle, so that the entire process of molding and molding can be high-speed and automated. A new type of molding machine with gas impact pressure, using the thixotropy of clay slurry, can obtain a very compact mold by instantaneously applying a pressure of 0.5 MPa. These developments are important conditions for the clay green sand casting to adapt to the requirements of modern industry. Therefore, this traditional process has been used to produce a large number of high-quality castings.

Dry Clay Sand

Clay dry sand molds have a slightly higher wet moisture content than those used in the production of this sand mold. After the sand mold is made, the surface of the cavity should be coated with refractory paint, and then placed in an oven for drying, and after it has cooled down, it can be molded and poured. Drying clay sand molds takes a long time and consumes a lot of fuel, and the sand molds are easily deformed during the drying process, which affects the accuracy of castings. Clay dry sand molds are generally used to make steel castings and larger iron castings. Since chemically hardened sand has been widely adopted, dry sand types have tended to be eliminated.

Chemical Hardening

Chemical hardening sand The molding sand used in this type of sand is called chemical hardening sand. The binder is generally a substance that can polymerize molecules and become a three-dimensional structure under the action of a hardener, and various synthetic resins and water glass are commonly used. There are basically 3 ways of chemical hardening.

  1. Self-hardening: The binder and hardener are both added during sand mixing. After the sand mold or core is made, the binder reacts under the action of the hardener to cause the sand mold or core to harden by itself. The self-hardening method is mainly used for modeling, but it is also used to manufacture larger cores or cores with small production batches.
  2. Aerosol hardening: Add binder and other auxiliary additives when mixing sand, do not add hardener first. After modeling or core making, blow in the gaseous hardener or the liquid hardener atomized in the gaseous carrier to disperse it in the sand mold or core to cause the sand mold to harden. The aerosol hardening method is mainly used to make cores, and sometimes it is also used to make small sand molds.
  3. Heat hardening: Add binder and latent hardening agent that does not work at room temperature when mixing sand. After the sand mold or core is made, it is heated. At this time, the latent hardener reacts with certain components in the binder to generate an effective hardener that can harden the binder, thereby hardening the sand mold or core. The heating hardening method is mainly used to make cores in addition to the manufacture of small thin-shell sand molds.

Waste Sand Regeneration

Since green sand has many advantages such as low cost, high efficiency, and pollution-free, it is still the main molding process of sand casting, and its castings account for more than 70% of the total castings. However, after the green sand is poured, the failed (dead) clay is sintered at a high temperature and wrapped on the surface of the sand grains, forming a firm “fish-pebbleized” ceramic layer. The direct use of waste (old) sand will greatly reduce the refractoriness of the molding sand, resulting in serious sand sticking on the surface of the casting. Therefore, the waste (old) sand must be treated by regeneration technology before it can be reused. Especially in recent years, resin core sand is mixed with green sand, and the mixed waste (old) sand is regenerated by a general mechanical rubbing method. The quality of the regenerated sand is difficult to reach the level of new sand. Therefore, it is urgent to seek a new regeneration process. In order to solve the regeneration problem of mixed waste sand of wet clay sand mold and organic resin sand core, a low-temperature thermal method + mechanical regeneration + chemical regeneration process was developed. The reason why foundry waste (old) sand can achieve high quality and low energy consumption by low-temperature roasting regeneration technology is mainly due to the use of the principle of secondary roasting. The heat exchanger is used to increase the temperature of the air entering the furnace, so that the carbon residue in the waste sand can be burned more effectively in the high-temperature heated air, and the generated combustion heat can also be used as a roasting heat source. The hot sand continues to flow in the heat exchanger for a long time, the unroasted carbon residue in the waste sand continues to burn, and the heat of combustion is absorbed by the heat exchanger, thus saving energy again. The carbon residues in the waste sand are burned out in the second roasting to achieve complete regeneration

Footnotes

  1.    Footnote 01

    Li Changhe Yang Jianjun. Metal Technology. No. 16 North Chenggen Street, Donghuang, Beijing: Science Press, first edition in May 2014. Fourth printing in July 2016: 34~36.

  2.    Footnote 02

    Chen Qiaosheng. A three-layer sand box for sand-turning foundry in a copper furnace and its casting process: CN, CN103008560A[P]. 2013.

  3.    Footnote 03

    Wu Daowei. Comprehensive utilization of raw materials for foundry foundry from coal slime[J]. Mineral Resources Conservation and Utilization, 2000(4).

  4.    Footnote 04

    Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 5 Casting Process. Beijing: Mechanical Industry Press, 2003

  5.    Footnote 05

    Li Hongying, Zhao Chengzhi. Casting process design: Machinery Industry Press, 2005

  6.    Footnote 06

    Ai Yunlong and so on. Engineering materials and forming technology: Machinery Industry Press, 2016: 202

  7.    Footnote 07

    Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 6 Special Casting. Beijing: Mechanical Industry Press, 2003

  8.    Footnote 08

    Sun Xiaoliang, Ma Zhiying, Pei Xiaohu, etc. Design and Numerical Simulation of Green Sand Casting Process for Gray Cast Iron Automobile Brake Discs. “CNKI; WanFang”, 2011

Footnote 01

Li Changhe Yang Jianjun. Metal Technology. No. 16 North Chenggen Street, Donghuang, Beijing: Science Press, first edition in May 2014. Fourth printing in July 2016: 34~36.

Footnote 02

Chen Qiaosheng. A three-layer sand box for sand-turning foundry in a copper furnace and its casting process: CN, CN103008560A[P]. 2013.

Footnote 03

Wu Daowei. Comprehensive utilization of raw materials for foundry foundry from coal slime[J]. Mineral Resources Conservation and Utilization, 2000(4).

Footnote 04

Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 5 Casting Process. Beijing: Mechanical Industry Press, 2003

Footnote 05

Li Hongying, Zhao Chengzhi. Casting process design: Machinery Industry Press, 2005

Footnote 06

Ai Yunlong and so on. Engineering materials and forming technology: Machinery Industry Press, 2016: 202

Footnote 07

Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 6 Special Casting. Beijing: Mechanical Industry Press, 2003

Footnote 08

Sun Xiaoliang, Ma Zhiying, Pei Xiaohu, etc. Design and Numerical Simulation of Green Sand Casting Process for Gray Cast Iron Automobile Brake Discs. “CNKI; WanFang”, 2011

On this page